The present invention relates to vacuum type circuit interrupters and more particularly to a method for forming the contact structure which is a part of such vacuum interrupters. This application discloses an improved method for manufacturing a chromium copper contact for use in a vacuum circuit interrupter.
Vaccum type circuit interrupters generally comprise an evacuated insulating envelope having separable contacts disposed within the insulating envelope. The contacts are movable between a closed position in which the contacts are engaged and an open when the contacts are separated and an arcing gap is established therebetween. An arc is initiated between the contact surfaces when the contacts move into or out of engagement while the circuit in which the interrupter is used is energized.
When the contacts are brought together the arc that is formed melts and vaporizes some contact material. After the contacts are brought together under high pressure engagement welds may be formed between the contact surfaces due to the melted contact material formed during arcing. Current surges also occur in the first few milliseconds of contact closing and these can also cause contact welding. The magnitude of the force required to break the weld so that the contacts can be opened depends upon many factors including the arc voltage and current, the contact area, and the contact material. These welds are objectionable since they interfere with the easy movement of the separable contacts and may result in the failure of the vacuum interrupter to open.
Another difficulty that is sometimes encountered with vacuum interrupter contacts is that materials used have excessive tendency to chop under low current conditions. This sharp chop in current can induce extremely high voltages across inductive devices connected in the circuit being interrupted, and such overvoltages can lead to destruction of circuit components. For an effective vacuum interrupter there should not be an excessive current chop on circuit opening.
It has been determined that an arc rotating contact formed from a 50% porous chromium matrix that is copper infiltrated is desirable for use in a vacuum interrupter. Approximately a 1/1 Cr-Cu ratio in the finished contact has been established as developing a low resistance contact having low strength weld and arc quenching characteristics necessary for a vacuum interrupter. The low compacting pressure, approximately twelve tons per square inch, necessary to produce a 50% dense chromium powder compact yields a compact having a very low green strength which cannot be ejected from a die without falling apart. Therefore, the compaction and sintering have to be carried out in a containment vessel for the copper during infiltration prior to machining to shape. For a normal spoked arc rotating contact, extensive machining is required to achieve the radial slots, the rimmed hole for the connecting rod and the intricate contact area. Heat generated by the extensive machining can also cause contamination of the contact because of the high affinity chromium has for nitrogen. To reduce manufacturing cost and to improve productivity it is desirable to have a process whereby a vacuum interrupter chromium copper contact can be pressed to the desired final shape.